ScienceDaily (Mar. 7, 2007) — The idea of "bone quality" is now widely used by physicians to predict which patients are at risk of fractures, but the idea has "major conceptual flaws" argues a team of researchers from Tampere, Finland in PLoS Medicine.

The concept of bone quality arose, say the researchers, as a way of explaining two phenomena.

First, drugs that act on the bone ('bone-targeted medication')-such as a class of drugs called "antiresorptive drugs," like alendronate and risedronate-can reduce fractures and yet this reduction in risk does not occur through the drugs changing the bone's density. This led to the theory that the reduction occurs by these drugs affecting some other feature of the bone, which came to be called "bone quality."

Second, "bone quality" has been offered as a solution to the "classic paradox of osteoporosis," the authors state. The paradox is that while low bone density values are associated with increased relative risk of fracture at the population level, the predictive value of bone density in an individual patient remains quite marginal. Perhaps, then, there is some other feature about the bone-the "bone quality"-that predicts an individual's fracture risk.

Despite the attractiveness of the concept of bone quality, the researchers point to several flaws in the concept. For example, we do not have a precise definition of bone quality, nor do we have an established mechanism for measuring it, and we don't even have criteria for defining "good" or "bad" bone quality.

"In the end," say the authors, "the only reasonable mechanism by which any bone-targeted medication reduces fractures is through increasing the whole bone strength one way or another."

"Accordingly, if we were able to accurately determine whole bone strength of individuals on antiresorptive therapy, the alleged discrepancy underlying the concept of bone quality would not exist."

Citation: Sievänen H, Kannus P, Järvinen TLN (2007) Bone quality: An empty term. PLoS Med 4(3): e27. (http://dx.doi.org/10.1371/journal.pmed.0040027)

Bone Fractures, Car Accidents, and Direction of Impact. Analogous to automobiles designed to run on their wheels, the human skeleton is adapted to bipedal gait and the resulting habitual locomotive loadings (Figure 1A). In terms of safety, the design of the cars is optimized to keep the drivers and passagers in the cockpit intact during collisions from the typical directions of impact, the front-rear directions (Figure 1B). However, a considerably smaller force can cause profound damage to the cockpit if subjected from atypical (unforeseen) direction (Figure 1C). Similarly, the capacity of the skeleton to resist fracture during accidents is generally good when the loading experienced during a traumatic incident is a moderate magnification of the loading experienced during habitual activities (i.e., within the inherent safety margin of bone), except in some c ases exceeding the bones' capacity to withstand the loading without breaking (Figure 1B). However, in many cases of older adults' fractures, the loading is completely different from that the bones are customarily adapted to (Figure 1C). (Credit: Image courtesy of Public Library of Science)

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